The present invention relates generally to overcurrent relay circuits, and more specifically, the present invention relates to overcurrent relay circuits having an internally produced power supply and a variable time-current characteristic circuit. In an overcurrent relay, a relay trip coil is energized a certain time after the current in the line being monitored exceeded a minimum trip level. The period between the time at which the minimum trip level is exceeded and the time at the trip coil is energized will depend upon the amount by which the trip level is exceeded. Energization of the trip coil is achieved by an appropriate power supply.
In prior art devices, there have been two basic approaches to providing the power supply for energizing the relay trip coil. One of these approaches utilizes the signal developed in the overcurrent relay circuit by placing a breakdown device, such as a Zener diode, and an associated series resistor in parallel with the test or load resistor. With such an arrangement, the test resistor must be relatively small, so that as much of the test current as possible will flow through the test resistor. However, the use of such a relatively small test resistor, plus the fact that a certain amount of current is still going to be bypassed through the Zener diode circuit, results in an inevitable inaccuracy or distortion in the correspondence between the voltage developed across the test resistor and the current in the line being monitored. Also, there is an undesired delay in operation arising from the necessity of developing an IR drop across the test resistor of sufficient magnitude to break down the Zener diode.
Another prior art approach, which avoids the problem of inaccuracy, is the utilization of a separate battery as the power supply for the relay trip coil. This approach, of course, has an economic disadvantage associated with the cost and maintenance of a separate battery.
Another problem associated with many prior art overcurrent relay circuits is that a single fixed time-current characteristic (TCC) circuit is used. Therefore, when it is desired to increase or decrease the time span between the current in the line being monitored reaching the minimum trip level and energization of the relay trip coil, the time amount characteristic circuit must be reconfigured. Therefore, line current conditions can result in numerous unnecessary line openings, or, conversely, an overload condition may exist for a longer time than desired prior to opening of the line. Since variable capacitors, fundamental elements of solid state analog overcurrent relays of the size that would be appropriate, are either not commercially available, or those available are cost prohibitive, some other approach is required to adjust the time-current characteristic.
There have been attempts to modify the time-current characteristics of overcurrent relays, such as the system illustrated in U.S. Pat. No. 3,167,636, assigned to the assignee of the present invention. However, systems of the type shown therein, involve extra switching which further complicates the system. Also, in some cases, variable value charging elements have been suggested. However, as indicated above, this type of element is impractical and subject to difficulties arising from insufficient accuracy over the large range of values needed.